>who_

The who command displays a list of all logged in users/display info
about current user
The who utility displays a list of all users currently logged on, showing
for each user the login name, tty name, the date and time of login, and
hostname if not local.

Flags

Here are just some of the most useful options for this command:

-b: Display time of last system boot.

-d: Print dead processes.

-H: Write column headings above the regular output.

-m: Only print information about the current terminal.

-q: Quick mode: List only the names and the number of users currently logged on.
When this option is used, all other options are
ignored.

-s: List only the name, line and time fields. This is the default.

-T: Print a character after the user name indicating the state of the
terminal line: "+" if the terminal is writable; "-" if it is not;
and "?" if a bad line is encountered.

-u: Print the idle time for each user, and the associated process ID.

am I: Returns the invoker's real user name. You can also use whoami

file: By default, who gathers information from the file /var/run/utmpx.
An alternative file may be specified.